Radiofrequency oscillators integrate a magnetoresistive device within which a spin-polarized electrical current flows. In such an oscillator, the passage of the current prompts a periodic variation in the resistance of the magnetoresistive device. A high-frequency signal, i.e. a signal whose frequency typically ranges from 100 MHz to some tens of GHz, is built from this periodic variation. The period of the variations of the resistivity, and therefore the oscillation frequency, can be adjusted by playing on the intensity of the current that crosses the magnetoresistive device and/or an external magnetic field.
Such oscillators are intended for example for use in radio telecommunications because they can generate a wide range of frequencies with a high qualify factor.
The term “quality factor” designates the following ratio:Q=fosc/Δf     Where:    Q is the quality factor,    fosc is the oscillation frequency of the oscillator, and    Δf is the width at mid-height of the line centered on the frequency fosc in the power spectrum of this oscillator.
Certain radiofrequency oscillators are derived from spin electronics.
Spin electronics uses the spin of the electrons as an additional degree of freedom in order to generate novel effects. The spin polarization of an electrical current results from the asymmetry existing between the diffusion of the spin-up type conduction electrons (i.e. electrons parallel to the local magnetization) and spin-down type conduction electrons (i.e. electrons anti-parallel to the local magnetization). This asymmetry leads to an asymmetry in the conductivity between the two spin-up and spin-down channels, whence a sharp spin polarization of the electrical current.
This spin polarization of the current is the source of magnetoresistive phenomena in magnetic multi-layers such as giant magnetoresistance (Baibich, M., Broto, J. M., Pert, A., Nguyen Van Dau, F., Petroff, F., Etienne, P., Creuzet, G., Friederch, A. and Chazelas, J., “Giant magnetoresistance of (001)Fe/(01)Cr magnetic superlattices”, Phys. Rev. Lett., 61 (1988) 2472), or tunnel magnetoresistance (Moodera, J S., Kinder, L R., Wong, T M. and Meservey, R. “Large magnetoresistance at room temperature in ferromagnetic thin-film tunnel junctions”, Phys. Rev. Lett 74, (1995) 3273-6).
Besides, it has also been observed that passing a spin-polarized current through a thin magnetic layer can induce a reversal of its magnetization when there is no external magnetic field (Katine, J. A., Albert, F. J., Buhrman, R. A, Myers, E. B., and Ralph, D. C., “Current-Driven Magnetization Reversal and Spin-Wave Excitations in Co/Cu/Co Pillars”, Phys. Rev. Lett. 84, 3149 (2000).
Polarized current can also generate sustained magnetic excitations, also known as oscillations (Kiselev, S. I., Sankey, J. C., Krivorotov, L N. Emley, N. C., Schoelkopf, R. J., Buhrman, R. A., and Ralph, D. C., “Microwave oscillations of a nanomagnet driven by a spin-polarized current”, Nature, 425, 380 (2003)). By using the effect of the generation of sustained magnetic excitations on a magnetoresistive device it is possible to convert this effect into a modulation of electrical resistance that is directly usable in electronic circuits and that can therefore, as a corollary, act directly at the frequency level. The U.S. Pat. No. 5,895,884 describes various developments implementing the physical principle mentioned here above. It describes especially the precession of the magnetization of a magnetic layer through which a spin-polarized electrical current flows. The physical principles implemented as well as the terminology used are also described and defined in the patent application number FR 2 892 871.
The oscillation frequency of these radiofrequency oscillators is adjusted by playing on the Intensity of the current that crosses it and, if necessary, also on an external magnetic field.
Prior-art radiofrequency oscillators comprise:
a magnetoresistive device within which there flows a spin-polarized electrical current to generate an oscillating signal oscillating at an oscillation frequency on a output terminal, the device comprising a stack of at least:                a first magnetic layer, called a “reference layer”, capable of spin-polarizing the electrical current, and having a magnetization of fixed direction,        a second magnetic layer, called a “free layer”, the magnetization of which can oscillate when it is crossed by the spin-polarized current, and        a non-magnetic layer, called a “spacer” interposed between the two previous layers to form a tunnel junction or a spin valve,        
a current source to make a current of electrons flow in said layers perpendicularly to them,
a control terminal to control the frequency or amplitude of the oscillating signal,
an automatic control loop connected between the output terminal and the control terminal to apply a control signal to the control terminal in order to automatically lock a characteristic of the oscillating signal into a reference value.
The spacer forms a tunnel Junction when it is designed to create the phenomenon of tunnel magnetoresistance. As a variant, the spacer forms a spin valve when it is designed to create the phenomenon of giant magnetoresistance.
The present applicant knows oscillators in which the automatic control loop is a frequency-locked loop. Certain frequency-locked loops are also known as phase-locked loops or PLLs, Such a loop comprises a frequency divider for the generation, from the oscillating signal, of an oscillating signal of reduced frequency which is compared with the signal of a reference clock. The control signal is generated from the difference between the reduced frequency and the frequency of the signal of the reference clock. The control signal is built so as to modify the frequency of the generated oscillating signal in a sense that reduces this difference.
The control signal is then injected into a control terminal for controlling the frequency of the oscillating signal in intensity. Typically, this intensity control terminal is that of a current summing element which enables the control signal to be added to the constant direct current generated by the current source.
Prior-art oscillators work well but several improvements are desirable. For example, it is desirable to improve the quality factor of these radiofrequency oscillators. If is also desirable to reduce their consumption of electricity.
The invention seeks to improve prior-art radiofrequency oscillators on at least one of these points.